Re: hugepage related lockdep trace.

[Michal Hocko <mhocko@suse.cz>]

On Wed 24-07-13 11:44:28, Minchan Kim wrote:
> On Tue, Jul 23, 2013 at 04:01:20PM +0200, Michal Hocko wrote:
> > On Fri 19-07-13 09:13:03, Minchan Kim wrote:
> > > On Thu, Jul 18, 2013 at 11:12:24PM +0530, Aneesh Kumar K.V wrote:
> > [...]
> > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > > index 83aff0a..2cb1be3 100644
> > > > --- a/mm/hugetlb.c
> > > > +++ b/mm/hugetlb.c
> > > > @@ -3266,8 +3266,8 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> > > >  		put_page(virt_to_page(spte));
> > > >  	spin_unlock(&mm->page_table_lock);
> > > >  out:
> > > > -	pte = (pte_t *)pmd_alloc(mm, pud, addr);
> > > >  	mutex_unlock(&mapping->i_mmap_mutex);
> > > > +	pte = (pte_t *)pmd_alloc(mm, pud, addr);
> > > >  	return pte;
> > > 
> > > I am blind on hugetlb but not sure it doesn't break eb48c071.
> > > Michal?
> > 
> > Well, it is some time since I debugged the race and all the details
> > vanished in the meantime. But this part of the changelog suggests that
> > this indeed breaks the fix:
> > "
> >     This patch addresses the issue by moving pmd_alloc into huge_pmd_share
> >     which guarantees that the shared pud is populated in the same critical
> >     section as pmd.  This also means that huge_pte_offset test in
> >     huge_pmd_share is serialized correctly now which in turn means that the
> >     success of the sharing will be higher as the racing tasks see the pud
> >     and pmd populated together.
> > "
> > 
> > Besides that I fail to see how moving pmd_alloc down changes anything.
> > Even if pmd_alloc triggered reclaim then we cannot trip over the same
> > i_mmap_mutex as hugetlb pages are not reclaimable because they are not
> > on the LRU.
> 
> I thought we could map some part of binary with normal page and other part
> of the one with MAP_HUGETLB so that a address space could have both normal
> page and HugeTLB page. Okay, it's impossible so HugeTLB pages are not on LRU.
> Then, above lockdep warning is totally false positive.
> Best solution is avoiding pmd_alloc with holding i_mmap_mutex but we need it
> to fix eb48c071 so how about this if we couldn't have a better idea?

Shouldn't we rather use a hugetlb specific lock_class_key. I am not
familiar with lockdep much but something like bellow should do the
trick?

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index a3f868a..40a61f6 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -463,6 +463,8 @@ static struct inode *hugetlbfs_get_root(struct super_block *sb,
 	return inode;
 }
 
+struct lock_class_key hugetlbfs_i_mmap_mutex_key;
+
 static struct inode *hugetlbfs_get_inode(struct super_block *sb,
 					struct inode *dir,
 					umode_t mode, dev_t dev)
@@ -474,6 +476,7 @@ static struct inode *hugetlbfs_get_inode(struct super_block *sb,
 		struct hugetlbfs_inode_info *info;
 		inode->i_ino = get_next_ino();
 		inode_init_owner(inode, dir, mode);
+		lockdep_set_class(&inode->i_mapping->i_mmap_mutex, &hugetlbfs_i_mmap_mutex_key);
 		inode->i_mapping->a_ops = &hugetlbfs_aops;
 		inode->i_mapping->backing_dev_info =&hugetlbfs_backing_dev_info;
 		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
-- 
Michal Hocko
SUSE Labs

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